USCG Exam PracticeTwin-screw vessel handling
The largest single contributor to propeller side force on a conventional shaft-driven vessel is BEST described as which of the following?
- A. The discharge screw current acting directly on the rudder blade.
- B. Aeration of the uppermost blade as it passes through the water surface.
- C. The frictional wake layer that the hull drags along its bottom.
- ✓ Unequal blade thrust caused by the shaft angle giving the descending blade a higher effective pitch.Correct
Why D is correct
USCG Boat Crew Seamanship Manual §10-A identifies the largest contribution to propeller side force as the angle of the propeller shaft, which gives the descending blade a higher effective pitch and therefore more thrust than the ascending blade, resulting in unequal thrust around the arc that pushes the stern to one side.